Heat Seeking Laser Sheds Light On Tern

SPE/IADC 67729

Heat Seeking Laser Sheds Light on Tern

C.K. Woodrow, Shell U.K. Exploration and Production, and E. Drummond, Sensa

Copyright 2001, SPE/IADC Drilling Conference

This paper was prepared for presentation at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 27 February–1 March 2001.

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Abstract

This paper describes the deployment of an optical fibre distributed temperature system, in a well on the Tern Alpha platform, which provides a measure of temperature in the wellbore. The temperature is measured using pulsed laser optical time domain reflectometry technology. As with virtually all such ventures into the unknown utilising new technology, several surprising results have been encountered, spawning local interest in accelerating the use of this potentially useful reservoir management tool.

This paper also describes what distributed temperature profiling using optical time domain reflectometry is, what it's for, and why it adds value. The paper highlights the early data acquired from the recent deployment activities on a well drilled from the Tern Alpha platform located in the Northern North Sea.

The vision is to be able to measure the 'health' of the well using it's temperature as an indicator, rather than waiting for a problem to develop and have to 'diagnose' using 'post mortem' production logging activities.

Background

Temperature has always been used to measure wellbore parameters, usually by continuously monitoring the wellhead temperature, and periodically measuring the temperature downhole using production logging tools, run on slickline, or electric line, and/or coiled tubing. More recently, additional temperature data has been provided at specific points in the tubing string downhole in some wells, as temperature is a pre-requisite for calibration of permanent downhole pressure gauges and/or for monitoring of electric submersible pumps.

The use of distributed temperature systems (DTS) utilising optical time domain reflectometry, provides for the first time, the ability to measure the temperature every metre down a well, or along a flowline or pipeline, on a permanent basis in real time.

Several vendors offer distributed temperature sensing systems utilising various fibre optic deployment methods.

Optical time domain reflectometry has been available as a commercial technology for well over a decade. However, deployment in oil and gas wells has been historically been limited to niche applications such as tertiary steam drive projects, where there is an obvious thermal profile coupled with a clear business case relating to the value of knowing where the steam front is located. Since 1995 there have been several steam flood well bore installations, and some have been operating at temperatures up to 280ºC.

Recently there have there been deployments in oil fields where cold water injection water can be detected 'breaking through' into oil producers see Ref. 1.

There have also been deployments with sand control screen assemblies, aiming to identify continuous inflow profiles, and early identification of damaged zones and/or gas or water breakthrough along the producing intervals of the wellbore see Ref. 2.

In most cases to date, there have been few deployment problems, and generally reliable signal processing, hence the overall distributed temperature systems available seem to be reasonably robust for wellbore deployment.

Following an analysis of the business case for this technology, it became apparent that a technology prover trial deployment should be carried out to confirm the viability of this potentially useful reservoir management tool. Several design modifications to the land based wellhead termination hardware were carried out to provide enhanced wellhead integrity essential to meet North Sea health, safety and environmental standards.

This paper highlights the deployment of a distributed temperature system in Tern Platform gaslifted well A27, which was subsequently converted to a water injector.

Principle of Operation

Installation of the distributed temperature Sensor is basically very simple - the well is equipped with a 1/4 inch O.D. control line. This can be installed either outside or inside casing/liners or screens. Specially coated optical fibre is then pumped into the control line and connected to an opto-electronic surface read-out unit. This was the optical fibre deployment technique utilised on the Tern Platform. This means that in the unlikely event of optical fibre degradation over time, it is possible to retrieve and replace the fibre sensor, without well intervention.

The coated optical fibre can also be deployed externally on the outside of the tubing without pumping, simply protected by a braided wire sheath or control line. This technique usually involves splicing the fibre in the vicinity of the drill floor, however, this deployment method was not used in Tern due to the relatively high rig day rate. The fibre can also be run in and out of the well using typical braided 'electric' wireline techniques; however, North Sea sub-surface safety valves preclude permanent deployment of the fibre using this method.

Temperature can be measured every metre along a multi-mode optical fibre by sending 8ns pulses of 1064nm wavelength laser light down the fibre and analysing the back-scattered light spectrum from each metre interval.

Vibration of the molecules in optical fibre is related to the fibre’s temperature, and this causes weak back scattering of the laser light. Analysis of the Raman Stokes and Anti-Stokes

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